This invention relates to the chemical etching of aluminum capacitor foil to produce a fine etch structure using a sodium chloride etchant solution containing at least an equal amount of sulfuric acid, preferably up to 22 wt%. Etching is carried out at 90.degree. to 105.degree. C.
The use of sodium chloride solutions to etch aluminum is well-known. Frequently, the solutions are acidified with strong acids. When such solutions are used to chemically etch aluminum without the use of electricity, a heavy metal compound such as iron, copper or nickel compound is added to catalyst the reaction. However, if traces of these metals remain on the surface or within the pores of the etch structure, these trace amounts can later interfere with the operation of a capacitor made from such foil.
Thus, it is highly desirable to improve the chemical etching of aluminum capacitor foil to produce a fine etch structure without resorting to the addition of heavy metal salts. It is also desirable to obtain the fine etch structure within a reasonable time period and at a reasonable temperature.